if you're going with this gpu long term (>3, maybe 4 years) go AMD, they have been fantastic at long term driver support and through history their gpus are weaker then nvidias at launch but due to how well they support their gpus they end up way stronger then when you first got them. the 6950 is still receiving game optimizations and full driver support to this day.
nvidia does a pretty poor job at supporting their gpus over the long term but for the first 2-3 years of a gpus lifespan they will be fantastic and if you plan on replacing the GPU in around that time span then you would go nvidia. usually past the 2-3 years of your nvidia gpu lifespan they usually cut off most game optimizations and you'll mostly receive bugfixes for your driver updates.
basically, AMD long term, nvidia short term.